A MTJ memory element is also referred to as a MTJ cell or MTJ and is a key component in magnetic recording devices, and in memory devices such as magnetic random access memory (MRAM) and spin torque transfer (STT)-MRAM. The fabrication method and the integration scheme with a complementary silicon oxide semiconductor (CMOS) substructure are two of the critical factors required for successful commercial production of MRAM. This new kind of non-volatile memory will be a replacement for DRAM, SRAM and flash. In MRAM design, the memory element is the magnetic tunnel junction (MTJ) that consists of two ferromagnetic layers separated by a thin insulating layer referred as a tunnel barrier layer. One of the ferromagnetic layers has perpendicular magnetic anisotropy (PMA), or is pinned by an antiferromagnetic layer set to a fixed magnetic moment in-plane orientation. The orientation of the other ferromagnetic layer referred as a free layer, is free to switch between a parallel and antiparallel direction to the pinned layer. When the magnetic moments of the two ferromagnetic layers are parallel, the resistance of the MTJ is lower compared with an antiparallel orientation, and these two orientations correspond to two memory states. The resistance of a MTJ cell is measured by powering the corresponding transistor which flows current from a bit line through the MTJ to a source line or vice versa. The magnetoresistive ratio is expressed by dR/R where dR is the difference in resistance between the two memory states when a current is passed through the MTJ, and R is the minimum resistance value.
An important step in fabricating an array of MTJs on a substrate is etch transfer of a pattern in an overlying hard mask through a MTJ stack of layers to form a plurality of MTJ cells with a critical dimension d that in state of the art devices is substantially less than 100 nm from a top-down view. Some of the MTJ layers have a thickness as small as 10 to 20 Angstroms. As shown in FIG. 1, the MTJ stack typically has a seed layer 21, pinned layer 22, tunnel barrier layer 23, free layer 24, cap layer 25, and hard mask layer 26 that are sequentially formed on a bottom electrode 11. The cap layer may be a metal oxide to enhance PMA in the free layer while the hard mask is usually a metal such as Ta that serves as a protective layer during subsequent physical and chemical etches. The bottom electrode also known as a bit line is insulated from other bit lines (not shown) by insulation layer 12, and is formed on a CMOS substructure 10 generally comprised of transistors, vias, and other components. MRAM device fabrication requires patterning the MTJ stack by one or more reactive ion etch (RIE) or ion beam etch (IBE) steps. First, a photo mask pattern 27 having a critical dimension d is formed on the hard mask. A first etch step is employed to transfer the shape in the photo mask through the hard mask thereby forming sidewall 26s. 
Referring to FIG. 2, the hard mask 26 serves as a protective mask during one or more etch steps that transfer the shape and critical dimension in the hard mask through the remaining layers in MTJ stack thereby forming MTJ cell 20n. The thin MTJ layers are easily damaged proximate to sidewall 26s that extends from a top surface of the hard mask to top surface lit of the bottom electrode. Moreover, redeposition of a metal layer 30 along the sidewall of the MTJ due to the non-volatile nature of etch by-products leads to shorting around the tunnel barrier layer 23, for example. Thus, MRAM device performance is degraded or may become non-functional. Accordingly, the most important challenge for MRAM cell fabrication is the patterning of the MTJ stack without damaging or shorting the device.
The etch transfer process through the MTJ stack of layers is challenging since there are a variety of materials (magnetic alloys, non-magnetic metals, and dielectric films) that each have a different etch rate when subjected to IBE with Ar or to conventional CH3OH based RIE. Care must be taken to select a hard mask 26 with a substantially lower etch rate than underlying layers in MTJ stack 20. Moreover, methanol RIE causes chemical and plasma damage on MTJ sidewalls although there is minimal redeposition of etched material on the sidewalls. For large device sizes, the damaged area is minor compared with non-damaged area and may not be critical. However, with the scaling down of the device size where d is below 100 nm, the amount of damaged area will become significant and degrade the magnetic properties of the MTJ. On the other hand, IBE produces no chemical damage and leaves minimal plasma damage, but results in a high degree of redeposited material on MTJ sidewalls. Redeposition at the MTJ sidewall is the key concern for IBE, especially for high density arrays with limited pitch between the MTJ cells. Increased cell density will limit the incident angle of the ion beam and possibly leave the redeposited material at the sidewall causing shorting of the devices.
Therefore, an improved method for fabricating an array of MTJ cells is needed that avoids patterning a hard mask on a MTJ stack of layers, and subsequent etching through the underlying MTJ stack. In particular, a method is desired that enables MTJ patterning without subjecting the sidewalls to ionic or chemical species that can damage the MTJ layers.